Proto-oncogenes encode proteins that comprise a select group of cellular regulatory proteins whose mutation or aberrant expression can result in oncogenic transformation. With the exception of certain growth factors and their receptors, the definition of normal function for most proto-oncogene products has been elusive. Our discovery that a member of the src-family of tyrosine protein kinases (the lck gene product) is associated with both the CD4 and CD8 T-lymphocyte surface glycoproteins provides the first clue to understanding the potential physiological functions of this family of proto-oncogenes. The results of our work over the past year have defined the sequences of the lck protein, which are important for the association with the CD4 and CD8, and have demonstrated that the CD4/CD8-lck complexes in T-cells are capable of transducing signals into the T-cells, which play a role in T-cell activation. These results imply that the CD4/CD8-lck complexes are structurally distinct but functionally similar to other tyrosine protein kinases that function as growth factor receptors. Our results imply that other members of this family may possess similar functions.